1. Field of the Invention
The present invention generally relates to optical communication systems, transmission deterioration compensation methods, optical transmission systems, and optical receiving systems, and more particularly to an optical communication system, a transmission deterioration compensation method, an optical transmission system, and an optical receiving system, which a transmission deterioration of an optical signal is compensated.
2. Description of the Related Art
For example, in an optical communication system, a WDM (Wavelength Division Multiplexing), which is a method for multiplexing a plurality of data signals having different wavelengths into a single optical fiber, has been used. The WDM (Wavelength Division Multiplexing) conducts a multiplex transmission with an optical signal by utilizing a plurality of optical beams having different wavelengths as a carrier in that an optical coupler (multiplexer) is arranged at a transmission side of the optical fiber and an optical branching filter (demultiplexer) is arranged at a receiver side of the optical fiber.
Recently, in optical communication systems, the WDM, in which a modulation rate is 10 Gbps for each wavelength, is in practical use. In order to further extend a transmission capacity, a WDM having 40 Gbps or 80 Gbps of the modulation rate for each wavelength is further required.
However, in an optical communication system for transmitting an optical signal of the modulation rate exceeding 40 Gbps, a transmission deterioration of a distortion of a waveform of the optical signal caused by a polarization mode dispersion (PMD) and a polarization dependent loss (PDL), which are rarely problems in the meantime, becomes a problem.
The polarization dispersion causes the distortion of the waveform of the optical signal. On the other hand, in general, the polarization mode loss rarely becomes a problem. However, when the polarization mode loss is accumulated, a polarization direction of the optical signal fluctuates. Accordingly, a level of the optical signal fluctuates due to an optical circuit from a light emitting element to a light receiving element, and polarization dependence.
In a conventional optical communication system, a method for compensating the polarization dispersion of the optical signal after a transmission at a previous stage of a receiving part by arranging a PMD/PDL compensator at a receiver side (for example, refer to Japanese Laid-open Patent Application No. 7-221705), a method for replacing a transmission itself with a special optical fiber, which does not cause the polarization dispersion, (for example, refer to Japanese Laid-open Patent Application No. 8-54525) and a like are used.
FIG. 1 is a diagram showing a system configuration example of an optical communication system for transmitting a single wavelength with a single optical fiber. In an optical communication system in FIG. 1, a terminal station A and a terminal station B are connected to each other via a relay station A and a relay station B. Each of the terminal station A and the terminal station B includes an optical transmitting system for transmitting an optical signal and an optical receiving system for receiving the optical signal.
Each optical receiving system of the terminal station A and the terminal station B includes a PMD compensation means for compensating a polarization dispersion, and a PDL compensation means (for example, an optical amplifier for conducing an output constant control) for compensating a polarization dependent loss. However, the optical communication system in FIG. 1 has a problem in that a receiver side is required to always measure a change of a polarization state and complicatedly control the PMD compensation means and the PDL compensation means.
FIG. 2 is a diagram showing a system configuration example of an optical communication system for transmitting dual wavelengths with a single optical fiber. FIG. 3 is a diagram showing a configuration example of a dual wavelength WDM multiplexer. Similar to the optical communication system in FIG. 1, in the optical communication system in FIG. 2, a terminal station A and a terminal station B are connected to each other via a relay station A and the relay station B.
Each of the terminal station A and the terminal station B has the dual wavelength WDM multiplexer shown in FIG. 3. In the dual wavelength WDM multiplexer, a PMD/PDL compensator for compensating the polarization dispersion and the polarization dependent loss is arranged for each wavelength at the receiver side.
However, in the optical communication system in FIG. 2, it is required to arrange the PMD/PDL compensator for each wavelength at every predetermined distance. However, in an optical communication system having a modulation rate of 40 Gbps or 80 Gbps for a single wavelength, compared to the optical communication system of 10 Gbps, a system cost is expensive. Accordingly, it is difficult to arrange the PMD/PDL compensator for each wavelength at every predetermined distance. In addition, there is a problem in that a setting of an optical fiber for a transmission channel is further expensive.
Moreover, in a case in that the polarization state rapidly changes, since the optical communication system is required to follow the change of the polarization state and control the polarization dependent loss, disadvantageously, the receiver side is required to measure the change of the polarization state for each wavelength and complicatedly control the PMD/PDL compensator.